Activated carbon is a versatile and inexpensive adsorbent produced from a variety of abundant carbon-containing raw materials, such as coal, wood and coconut shells. The unique properties of activated carbon relate to the carbon-based backbone, which is highly porous over a broad range of pore sizes from visible cracks and crevices to cracks and crevices of molecular dimensions. Intermolecular attractions in these smallest pores result in adsorption forces, which cause condensation of adsorbate gases or precipitation of adsorbates from solution into these molecular scale pores.
Activated carbon, once manufactured, is generally used to adsorb organic compounds from liquid and vapors streams. Activated carbon is also used to a much lesser extent as a catalyst support, whereby the activated carbon backbone serves to stabilize and immobilize a catalytic material. The catalytic material is typically a metal compound and the subsequent “catalyst” is usually used under reducing conditions, in the absence of significant molecular oxygen, to catalyze additions of hydrogen to organic compounds. It is not common industrial practice to utilize activated carbon as a catalyst support for reactions involving oxygen or a source of molecular oxygen such as air, due to concerns that the carbon backbone will enter into combustion reactions and destroy the catalyst and associated equipment. The literature includes one process whereby hydrocarbons are adsorbed on “catalyst-impregnated activated carbon at ambient temperature, and then raising the temperature high enough to oxidize the adsorbate but not the carbon. Such a scheme requires a catalyst-substrate system in which the oxidation of the adsorbate and that of the carbon occur at significantly different temperatures.” (“Catalytic Oxidation of Adsorbed Hydrocarbons,” J. Nwankwo and A. Turk, Annals NY Acad. Sci. Vol. 237, pp 397–408 (1974)).
Activated carbon also serves as the support for many impregnating agents that chemically react with vapor phase contaminants. The most common example is impregnation with a caustic substance, such as sodium or potassium hydroxide, for the purposes of increasing capacity for the treatment of hydrochloric and sulfuric acid vapors. In this manner, the capacity for neutralizing the acid gases is greatly increased over unmodified activated carbon alone, which possesses relatively little native buffering capacity. Other impregnating agents are specific catalysts introduced into gas mask carbons, which catalyze the decomposition of specific chemical agents, such as phosgene.
Unmodified activated carbon does show reactivity towards molecular oxygen, including reactions in wetted carbon at ambient temperatures. This wetted carbon has been described for use in pH control in water treatment applications (U.S. Pat. No. 5,368,739). Another method of oxidizing activated carbon for pH control in water treatment applications involves conditioning virgin activated carbon in air at temperatures of 300 C to 700 C for between 5 minutes to 3 hours (U.S. Pat. No. 5,368,738). Activated carbon is also known to promote a few oxidation reactions under ambient conditions, such as the adsorption of hydrogen sulfide, followed by the oxidation to sulfuric acid in the presence of water vapor. There are also commercially available activated carbons specifically manufactured to have intrinsic catalytic properties to catalyze free radical reactions, such as the decomposition of hydrogen peroxide.
While activated carbon has shown a variety of diverse applications, the predominant application is adsorption for the removal of chemical species from waters and wastewaters, as well as the removal of chemicals from vapor streams, most commonly air. It is in these applications, where the activated carbon is being utilized without modification after activation of the carbonaceous raw material, that the novelty of the current invention resides. Specifically, this invention provides a new option for removing adsorbed oxidizable organic compounds from the internal structure of the activated carbon. This option can either be implemented as the activated carbon continues to adsorb additional compounds, thereby greatly extending the adsorption capacity of the activated carbon, or it can be applied following the adsorption process, thereby allowing the activated carbon to be regenerated and returned for additional adsorption service.
In normal adsorption applications, activated carbon gradually accumulates chemical species removed from the liquid or vapor stream being purified, causing a progressive reduction in the carbon's ability to remove additional chemicals from the stream being treated. At some interval, i.e., when the activated carbon has become “spent”, it must be replaced or regenerated to restore the adsorptive capacity. Depending on the effect of the regeneration process on the properties of the activated carbon, repeated regenerations may be possible on the same activated carbon, thereby greatly extending the useful life of the activated carbon in adsorption service.
Two methods of regeneration of spent activated carbon have found widespread industrial application: steam regeneration and thermal reactivation. In addition, there are several specialized techniques, such as solvent regeneration, chemical regeneration and super-critical fluid extraction, that have been utilized on occasion, but to a much lesser extent than the two mainstream regeneration methods.
Steam regeneration uses direct contact steam to strip the adsorbed organics away from the surface of the carbon and is routinely used for vapor-phase carbon. This technique exploits the phenomenon that the volatility of the adsorbed compounds increases with temperature. Thus, by increasing the temperature of the carbon, the equilibrium of the adsorbed chemicals can be shifted from condensed liquid in the internal pores of the carbon to the vapor phase, desorbing some of the adsorbate out of the carbon. This results in the regeneration of some of the carbon's capacity for subsequent adsorption.
Steam regeneration can successfully be utilized for volatile organic adsorbates with atmospheric boiling points up to about 120 degrees Celsius (120C). This method has the advantage that regeneration conditions are mild and the internal pore structure of the carbon is unaffected by the regeneration conditions. Unfortunately, only portions of the available adsorption pores are steam regenerated and less volatile compounds, if present, are not effectively removed and reduce the recovered adsorptive capacity of the carbon.
Sometimes, a hot inert gas such as nitrogen is used in place of steam. Steam and hot inert gases regenerate carbon in the same manner, by heating the carbon and volatizing adsorbates directly from the surface of the internal pores of the carbon. On other occasions, the activated carbon may be heated to temperatures as high as 500C under oxygen-depleted conditions, typically by using recycled flue gases. Under those conditions, the adsorbates decompose into gaseous fractions (such as volatile hydrocarbons, water vapor and oxides of carbon and nitrogen) and a carbonaceous residue or char, which forms within the pores of the activated carbon. Even these high temperature conditions do not appreciably deteriorate the original backbone of the activated carbon in the absence of appreciable molecular oxygen. However, depending on the adsorbates, the relative fraction of char deposited in the carbon pores will vary. The remaining char does consume recovered adsorption capacity and, in general, a slow poisoning of the carbon is observed over repeated regenerations.
Thermal reactivation involves heating the activated carbon up to temperatures above 800 degrees Celsius, restricting sources of molecular oxygen and introducing either steam or carbon dioxide as an oxidizing gas. Under those conditions, phenomena known as the “water-gas shift reactions” occur, which convert both deposited char and the graphitic backbone of the activated carbon into carbon monoxide (and hydrogen in the case of steam). The aggressive conditions of thermal reactivation effectively remove the deposited char. Unfortunately, some of the graphitic backbone of the carbon is also removed during thermal reactivation, leading to the gradual destruction of the internal pores and the eventual loss of adsorption capability and mechanical strength of the activated carbon. Furthermore, the carbon monoxide and hydrogen products of the water-gas shift reactions typically require further oxidization to form carbon dioxide and water vapor prior to release into the atmosphere.
One final regeneration method found in the prior art involves the “wet air oxidation” of spent activated carbon, where spent activated carbon is regenerated by oxidizing an aqueous solution containing a dispersion of a “carbonaceous surface adsorbent” containing adsorbed combustibles (U.S. Pat. No. 3,442,798). This technology is an example in the prior art where molecular oxygen alone is used to promote the oxidation of adsorbed organics contained within the pores of the spent activated carbon, thereby regenerating the activated carbon for reuse. Notably, the conditions provided for the wet air oxidation are such that combustibles also react directly with the molecular oxygen present in the aqueous solution. As noted in the text of this patent, column 3 lines 13–22: “A related aspect of the present invention is to use the process to obtain greater efficiency in wet air oxidizing combustibles and to wet air oxidize combustibles at lower temperatures than could be used if the oxidized combustibles were not concentrated by and adsorbed upon any surface enlarging agent. In other words, the use of a surface active adsorbent, which is not itself to be oxidized, allows the wet air oxidation of the combustibles concentrated thereon to proceed under lower temperature conditions or improved oxidation efficiencies.” As such, the contribution of the surface active adsorbent is limited to concentrating the oxidizable compounds and enhancing the reaction rates by providing increased localized concentrations of combustibles, with the reactions being the same reactions that would occur in solution, although slower, in the absence of the surface active adsorbent.
Irrespective of the specific method of regeneration, all techniques for the regeneration of spent activated carbon can be evaluated in terms of the source and amount of energy required, the extent that the adsorbed compounds are removed, the fate of the desorbed compounds, and the impact of the regeneration conditions on the original activated carbon internal pore structure, adsorption capacity and mechanical integrity.
Steam regeneration utilizes the least amount of energy of any regeneration method and that energy is provided as low-pressure steam. Unfortunately, steam regeneration is effective only for low boiling materials and only regenerates the lower energy adsorption pores, recovering 20 to 50 percent of virgin activated carbon adsorption capacity. The desorbed material is generally chemically unchanged, but mixed with water vapor and must be either recovered or treated. The principal limitation is that steam regeneration does not perform acceptably for vapor phase compounds that are less volatile, and rarely works acceptably for the regeneration of spent activated carbon from liquid adsorption applications. As such, the steam regeneration method is typically limited to solvent recovery applications, where the solvent can be recovered, purified and reused, or subsequently incinerated.
The hot inert gas methods and the higher temperature flue gas techniques have a common drawback that they are heating the spent activated carbon with a gaseous steam, which requires large volumes of heated gases to supply the necessary energy to raise the temperature of the activated carbon bed. This drawback is in contrast with steam regeneration, where the heat of condensation of the steam supplies the majority of the energy necessary to raise the temperature of the activated carbon bed. The lack of condensation results in large amounts of off gases from the hot inert gas and anoxic higher temperature techniques. Since the off gases are contaminated with the desorbed compounds or the decomposition products of the previously adsorbed compounds, the entire off gas stream requires subsequent treatment to avoid unacceptable air emissions. Furthermore, depending on the properties of the adsorbed compounds, increasing amounts of low volatility adsorbed compounds and/or char may accumulate in the pore structure of the activated carbon, resulting in a progressive loss of available adsorption capacity.
Thermal reactivation is the only currently available regeneration technology that effectively addresses the problem of accumulation of non-desorbed compounds and char in the pores of the activated carbon. Unfortunately, the conditions for the water-gas shift reaction are so aggressive that the activated carbon pore structure and graphitic backbone are attacked along with the removal of the char present. Furthermore, the extremely high temperatures require the greatest amount of energy of all the regeneration approaches. In addition, the water-gas shift reactions are endothermic, meaning that they absorb energy as the reaction proceeds, with this energy being consumed at the highest temperatures.
Wet air oxidation requires that the activated carbon be contained in an aqueous suspension under sufficient pressure to maintain the liquid phase and within a temperature range of 125C and the critical temperature of water (374C). Unfortunately, the solubility of oxygen in water within this temperature range is very low, resulting in significant engineering challenges to get the oxygen dissolved in the aqueous dispersion so the oxygen can diffuse into the internal pores of the activated carbon and oxidize the adsorbed organics. The combination of high pressures to facilitate dissolving the oxygen and maintaining the liquid solution without vaporization result in very high equipment and operating costs for wet air oxidation and prevent it from being a cost-effective method of regenerating spent activated carbon.